Nutrient Remineralization by Ramicrusta

Background

 

The genus Ramicrusta is a red calcifying macroalgal that colonizes any hard substrate, often overgrowing numerous living sessile organisms (Eckrich & Engel, 2013). Despite its ability to overgrow sponges, gorgonians, and crustose coralline algae, Ramicrusta has high interactions with reef-building corals (Edmunds et al., 2019). Coral genera exhibit varying levels of vulnerability to Ramicrusta colonization, with Orbicella sp. among the worst competitors and experience the highest alga overgrowth rates (Hollister et al., 2021).

 

Fig. 1: R. textilis overgrowing an O. annularis colony (photo: Abigail Gretta).

 

Considerable evidence illustrates the tendency of Ramicrusta to interact with and overgrow live corals, yet there is a disconnect when it comes to understanding the factors contributing to this relationship. Macroalgal growth is typically nutrient-limited, especially in oligotrophic tropical waters (Amato et al., 2016; Strait et al., 2021). Thus, Ramicrusta may be deriving nutrients from the coral itself, fueling these interactions and promoting its own growth.

Bulk stable isotope analyses were used to determine if Ramicrusta remineralizes coral nutrients as it overgrows O. annularis colonies. Differences in the nutrient profiles of the alga overgrowing live corals (O. annularis) versus the alga growing on rocks (control) would suggest remineralization is occurring.

The following are the preliminary methods, results, and conclusions from a pilot study conducted on September 2, 2024.

Methods

Site:

Sample collection occurred on the leeward side of Flat Cay, an offshore 9 m depth reef along the southwest side of St. Thomas, U.S. Virgin Islands (Fig. 2). This site was selected because of its high abundance of R. textilis (46.0% ± 5.9; Hollister et al., 2021) and high frequency of interactions between the alga and Orbicella annularis (personal observation). To characterize the abiotic conditions during algal and coral sampling, a temperature logger (Hobo Pendant) and PAR meter were deployed a month prior to the collection time and retrieved on the final day of sampling.

Fig. 2: Site

Alga Collection:

Open-circuit SCUBA was used for all algal collections. Replicates of each algal were randomly collected by hand, with a minimum of five meters between samples. R. textilis were brushed of epiphytes and chiseled from 10 rocks (control) and 10 O. annularis colonies. O. annularis colonies that appeared visually healthy were selected for alga collection. Two R. textilis samples were removed from each substrate (n = 20 R. textilis per substrate type), one at the margin of algal growth (n = 10) and the other at a linear distance away (2 to 3 cm) from the margin on the same contiguous algal thallus (n = 10), with distances between samples measured and photographed (Fig. 3). This sampling strategy will distinguish the alga’s nutrient parameters during direct interaction with living coral to those in non-interacting regions, with the rock samples serving as controls for both conditions. Algal replicates were placed in individual plastic bags at depth with ambient seawater. Samples were transported to the laboratory in a dark container to minimize physiological stress and processed immediately.

Fig. 3: R. textilis were collected from 10 O. annularis colonies (left) and 10 rocks (control), with marginal and non-marginal replicates sampled from each substrate type.

Lab Processing:

Macroalgae were rinsed with deionized water (DI) to remove any external contaminants (i.e., invertebrates and epiphytes) and thoroughly dried with paper towels (Strait et al., 2021). Samples were loosely parceled in combusted aluminum foil with each sample code and dried at 60℃ to constant weight in a drying oven (24-36 hours). With a clean mortar and pestle (ethanol and DI rinse between samples), algae were ground to a fine powder and carefully packaged in 1.5 mL Eppendorf tubes (Strait et al., 2021). Stable isotope analyses were processed by an Elemental Analyzer Delta V at the University of Hawai’i at Mānoa. Dual stable isotope analyses were performed because acidification has the potential to degrade \(\delta^{15}\)N and unaltering calcified tissue can skew \(\delta^{13}\)C (Strait & Spalding, 2021).

Graphs & Analyses

Data Analysis:

All data were tested for homogeneity and normality. Log transformations were used when data violated ANOVA assumptions. Tissue nutrients (\(\delta^{15}\)N, % N, and C:N) were assessed using two-factor ANOVA (factors: Substrate, Growth Region, and the interaction of the two). Tukey’s honestly significant difference post hoc test was applied to show pairwise comparisons, with distinct letters on graphs denoting significant differences.

Analysis of Variance Model

	Df	Sum Sq	Mean Sq	F value	Pr(>F)
Substrate	1	0.1896	0.1896	4.158	0.04978
Growth_Region	1	0.3313	0.3313	7.264	0.01112
Substrate:Growth_Region	1	0.08185	0.08185	1.794	0.1898
Residuals	32	1.46	0.04562	NA	NA

Analysis of Variance Model

	Df	Sum Sq	Mean Sq	F value	Pr(>F)
Substrate	1	0.49	0.49	0.8808	0.355
Growth_Region	1	8.258	8.258	14.84	0.0005287
Substrate:Growth_Region	1	1.491	1.491	2.681	0.1114
Residuals	32	17.8	0.5563	NA	NA

Analysis of Variance Model

	Df	Sum Sq	Mean Sq	F value	Pr(>F)
Substrate	1	56.95	56.95	11.05	0.00223
Growth_Region	1	0.6441	0.6441	0.125	0.726
Substrate:Growth_Region	1	0.03711	0.03711	0.007202	0.9329
Residuals	32	164.9	5.153	NA	NA

Conclusion